Peel and finely dice the onion and garlic. Melt the butter, sauté both in it. Pour in fish stock and wine, reduce to half over high heat.
Cut the fish into bite-sized pieces and drizzle with lemon juice. Peel the carrot and cut into fine strips. Clean the spring onions and cut into thin rings. Quarter tomatoes. Salt and pepper the fish and let it steep in the stock for 5 minutes. Lift out and keep warm.
Stir the cream and turmeric into the stock, reduce until creamy. Briefly simmer carrots, onions and tomatoes. Season to taste with mustard, honey, salt and pepper. Put the fish back in and let it steep for 2 minutes.